1. Field of the Invention
The present invention relates to a pixel structure and particularly relates to a multi-domain vertical alignment (MVA) pixel structure.
2. Description of Related Art
A liquid crystal display (LCD), with its good spatial utilization, low power consumption, no radiation, and other distinguished features, has become the mainstream of the market. For better display quality of the LCD, various LCDs with wide view angles have been developed on the market. Some common examples are an in-plane switching (IPS) LCD, a fringe field switching LCD, and an MVA LCD.
The MVA LCD provides an effect of a wide view angle. However, the light transmittance of the MVA LCD varies according to the view angles. That is, the brightness level of the MVA LCD differs in a normal vision and in a slant vision, thereby resulting in problems of color shift and low color saturation in the display image.
The problem of color shift often occurs in a slant vision in that Gamma curves of a normal vision angle and a slant vision angle are different (as illustrated in FIG. 1). FIG. 1 illustrates Gamma curves of normal vision angles and slant vision angles of an MVA LCD, wherein the horizontal axis represents gray level values and the vertical axis represents transmittance rates (T %).
In the current technology for solving the problem of color shift, sub-pixels in the pixels are coupled to different voltages so as to change tilt angles of the liquid crystals and to improve color shift. Two conventional technologies are briefly described hereunder.
Refer to US Patent No. US 2004/0001167 for the first conventional technology. FIG. 2 shows an equivalent circuit of a pixel in this conventional technology. As shown in FIG. 2, the conventional pixel comprises a transistor M21, a storage capacitor Cs21, and a liquid crystal capacitor Clc21, all three of which form a sub-pixel. The conventional pixel also comprises a transistor M22, a storage capacitor Cs22, and a liquid crystal capacitor Clc22, all three of which form another sub-pixel. Furthermore, there exist a parasitic capacitor Cgd21 and a parasitic capacitor Cgd22 in the pixel.
In the first conventional technology, the display voltage difference between the two sub-pixels is described by Formula 1:
                              Δ          ⁢                                          ⁢          Vlc                =                  Vcsp          *                      Ccs            Cpix                                              (        1        )            
Cpix refers to the sum of all the capacitance values of the pixel, Ccs refers to the capacitance value of the storage capacitor Cs21 or Cs22, and Vcsp refers to the alternating current (AC) voltage applied to the storage capacitor.
In the conventional technology, the display voltage difference is changed by adjusting a capacitance value Ccs and a voltage Vcsp of the storage capacitor. In other words, each of the two sub-pixels has a different pixel display voltage to improve the Gamma curves in a slant vision. As a result, the conventional technology provides a significant improvement. However, the demand for the RC delay of the Cs signals (i.e. the voltage applied to the storage capacitor, its value being Vcom±(+0.5)*Vcsp or Vcom±(−0.5)*Vcsp) is high. Extra space is required to dispose the conductive line of the Cs in the design of the peripheral area of the panel, which increases the panel area and affects the layout evaluation.
Refer to US Patent No. US 2005/0030439 for the second conventional technology. In this conventional technology, the two sub-pixels have different pixel display voltages to correct the problem of color shift in a slant vision. FIG. 3 shows an equivalent circuit of a pixel in the second conventional technology. As shown in FIG. 3, the conventional pixel comprises a transistor M31, storage capacitors Cs31 and Cs32, a coupled capacitor Ccp3, and liquid crystal capacitors Clc31 and Clc32.
The ratio of the display voltages of the two sub-pixels are shown in Formula 2:
                                          V            ⁢                                                  ⁢            32                                V            ⁢                                                  ⁢            31                          =                              Ccp            ⁢                                                  ⁢            3                                              Clc              ⁢                                                          ⁢              32                        +                          Ccp              ⁢                                                          ⁢              3                        +                          Cs              ⁢                                                          ⁢              32                                                          (        2        )            
However, the display voltage difference ΔVp of the two sub-pixels is inconsistent because of the coupled capacitor Ccp3. As a result, the common voltage Vcom of the two sub-pixels is also inconsistent. Accordingly, this results in an accumulation of electric charges and the condition of burn-in during a long period of display due to the lack of a dispersion path for the remaining electric charges.
Hence, a new pixel structure is provided herein to improve the problem of color shift in a slant vision and reduce the condition of burn-in without the need for special Cs signals.